1. Field of the Invention
A typical plasma display system comprises a plurality of elongate tubes formed from glass or other transparent material. The permanently sealed chambers are filled with any of the various gases which provide visible light upon ionization by the passage of an electric current through them. The tubes are arranged in a side by side and parallel fashion to form a flat viewing surface. A plurality of X electrodes are arranged and spaced apart in parallel relationship close to and preferably contacting the tubes along one side and generally orthogonal thereto. On the opposite side of the tubes, one of a plurality of Y electrodes is placed lengthwise along each tube. By applying a voltage of appropriate magnitude (usually 250-400 volts) between a selected X and a selected Y electrode, the gas in the chamber between the two selected electrodes can be made to ionize and conduct, the tube walls between the electrodes and the ionized gas volume acting as a capacitor to permit the current to flow. When the capacitive charge in the cell walls reaches a certain value, the voltage difference across the gas volume becomes insufficient to maintain further conduction, causing conduction and light emission to cease. It is well known that this wall charge will permit subsequent conduction by the cell wall in the opposite direction by the application of an appreciably lower opposite polarity voltage between these two electrodes. By applying between the plurality of X electrodes and the plurality of Y electrodes an alternating sustaining voltage, those gas volumes or cells which have been previously written (as defined by the presence of wall charge) can be maintained in that condition, and those not written or written and subsequently erased can be maintained in the unlit condition.
It is well known that for writing of individual cells to occur reliably and at a reasonably low voltage, at least one electron must be present in the gas volume to be written, since this allows a small amount of current to quickly avalanche into maximum ionization and current flow. It has further been discovered that the presence of such free electrons can be assured for a period of time by earlier writing of the chosen cell or another cell adjacent it in the same gas chamber. Even though wall charge is subsequently removed from the cell, free electrons will be available for a relatively long period of time, and permit subsequent writing at a relatively low voltage. One modification to create a supply of such free electrons involves the use of concealed pilot cells at the ends of the tubes constantly maintained in a lit condition. The use of such pilot cells is satisfactory for shorter chambers, on the order of a few inches long. However, for panels employing relatively long gas chambers in the 15-40 inch range, the pilot cells at the ends of the chambers cannot effectively create free electrons in the central parts of the chambers, with the result that writing of these centrally located cells cannot occur reliably.
2. Description of the Prior Art
One solution to this problem is described in U.S. Pat. No. 3,786,474 (Miller). As it is understood, Miller involves first placing a so-called condition pulse on the Y electrode associated with a gas chamber causing all unlit cells to light. Then a so-called X electrode neutralizing pulse is applied to all the cells along the tube except for those to be written, which causes all those lit by the conditioning pulse to be erased, without erasing those previously written. An addressing pulse is applied on the X electrode of the cell to be written with the result that this cell achieves written or lit status, because it is given the normal wall charge characteristic of such a state.
Another technique to solve this problem is to employ two different lit or conductive states to furnish the binary values required. Each state is represented by a different level or polarity of wall charge chosen so that a cell in one state can be switched to the other without disturbing other cells, so that all wall charge states can be sustained without affecting status of either. In such a situation, since all cells or gas volumes are undergoing periodic firing, no problem exists since free electrons are continuously created in the gas. See Data Manipulation and Sensing-Plasma Display, Robert L. Johnson, et al., December, 1971, dist. by NTIS, No. AD-737371, particularly Ch. II; Coordinated Science Laboratory Report for September, 1968, pub. Univ. of Ill. No. AD-692,196; and Coordinated Science Laboratory Report for July, 1969-June, 1970, Univ. of Ill., AD-711,278. Materials of High Vacuum Technology, Warner Espe, pub. Pergamon Press 1968 is a scholarly and authoritative work in the field.